Stormwater retention basins store water accumulated during a rain event and release the water at a controlled rate to prevent or limit downstream flooding and/or limit downstream waterway erosion.
Some basins have a discharge opening or orifice with a fixed cross sectional flow area located near the bottom of the basin. As the water level in the basin increases during a storm event, the rate of water discharged from the basin (which is essentially proportional to the square root of the water depth) increases with water level. Water is discharged from the basin at a maximum rate when the basin is at its most full condition (which is normally soon after the storm event has occurred).
Stormwater events often cause downstream flooding and also scour sediment from the bottom and sides of waterways. The greater the flow of water, the worse the problem becomes downstream. Site development, which tends to include impermeable surfaces such as parking lots, roofs, and the like, normally acts to increase the rate of site discharge and contributes to downstream flooding and erosion problems.
Typically, all of the sources of stormwater for a given waterway are discharging at their maximum rates during or shortly after a rain event. These sources include underground and aboveground stormwater storage systems, conventional stormwater collection systems, and also overland flows (sheet flows).
It is advantageous to use flow control devices to regulate the discharge of stormwater from stormwater basins and thereby manage the stormwater runoff of the waterway. It may be preferable to have flow control devices with different discharge profiles or discharge hydrographs at different stormwater basins. Some flow control devices may be configured so that the maximum discharge rate from some basins are delayed until after the maximum discharge rate from other basins. Yet other flow control devices may be configured so that the device can better discharge the initial inflow of water and then throttle discharge as the basin fills.
There is a need, therefore, for a flow control device that can be designed to achieve a specific engineered discharge profile.